There continues to be considerable interest in producing optical amplifiers for amplifying weak optical signals in both local and trunk optical networks. The high data rates and low optical attenuation associated with fiber optic lengths are well-established and continue to become more appreciated as fiber lengths become more economical compared with electrical coaxial cable alternatives. In spite of the relatively low magnitude of optical signal loss during transmission, the intrinsic linear attenuation law of lightwave energy in optical fibers necessitates optical repeater nodes to amplify and/or regenerate the digital optical bit streams or analog signals in long-haul terrestrial and undersea communication systems. Typically, unrepeated distances extend from 30 to 70 kilometers in length, depending upon the fiber loss at the selected transmission wavelength, which is ordinarily 1.31 or 1.55 microns, respectively.
One non-invasive approach to amplifying an optical signal in a fiber optic is presented in U.S. Pat. Nos. 4,955,025 and 5,005,175 entitled, "Fiber-Optic Lasers and Amplifiers" and "Erbium-Doped Fiber Amplifier," respectively. In these patents, a doped optical fiber is transversely coupled to a pump so that a weak optical input signal at some wavelength within the rare earth gain profile experiences a desired amplification. Pumping is effected by a separate laser or lamp which emits photons of appropriate energy, which is higher than that of the signal wavelength. Electrons in the doped fiber are excited from the ground state to one or more pump bands. The electrons then decay an amount corresponding to the wavelength at which the device operates. When a photon at the laser wavelength interacts with an excited atom, stimulated emission occurs. An output photon can thus originate from either previous spontaneous emission, stimulated emission, or an input signal.
Since erbium-doped amplifiers only operate at a specific wavelength, i.e., 1.53 .mu.m-1.55 .mu.m, other approaches to non-invasive optical amplifiers, operable for example at 1310 nm, are under investigation using semiconductor materials and variations of the rare-earth doped fibers. To date, however, serious problems have plagued development of these devices. Normally, semiconductor amplifiers provide low gain and require the fiber to be cut so that the signal can be extracted from and then re-launched into the optical fiber, while rare-earth doped amplifiers, like the Neodymium family of devices are unable to obtain sufficient gain at 1310 nm while minimizing inefficiencies due to lasing at the stronger optical transitions near 1064 nm and those due to pump-induced excited state absorption (ESA).
Thus, there exists a need in the optical telecommunications art for an improved optical amplifier and amplification method providing amplification characteristics commensurate with those attained by erbium-doped fibers at 1550 nm, but operable at any optical wavelength employed within an optical fiber. The present invention provides a compound waveguide architecture and amplification process to address this need.